The present invention generally relates to optical switching systems, and more particularly to an optical switching system used in an optical fiber communication and the like.
Various information interchange services such as a video conference system and a high definition television system are being realized using optical information networks such as integrated services digital networks (ISDNs). An optical switching system is used for making these information interchange services.
A conventional optical switching system employs a wavelength-division and time-division composite switch as a basic switch for a wavelength-division and time-division composite optical communication path. The wavelength-division and time-division composite switch is capable of arbitrarily interchanging the wavelength and time slot of an input optical signal which is a wavelength-division multiplexed and time-division multiplexed signal.
FIG. 1 shows a basic structure of the wavelength/time composite switch in an example of the conventional optical switching system. An optical input signal which is applied to a variable branching filter 1 is a wavelength-division multiplexed (wavelengths of .lambda..sub.1 through .lambda..sub.n) and time-division multiplexed signal. The variable branching filter 1 branches the input optical signal to k optical where k is an integer greater than or equal to one, for every predetermined wavelength. In addition, the selecting wavelength of the variable branching filter 1 is switched for every time slot of the time-division multiplexed signal component. Outputs of the optical T switches 2 are converted into predetermined wavelengths for every time slot in variable wavelength converting elements 3. Outputs of the variable wavelength converting elements 3 are combined and output as an optical output signal of the wavelength/time composite switch.
FIG. 2 shows an equivalent circuit of the wavelength/time composite switch shown in FIG. 1. As shown in FIG. 2, n time division highways, where n is an integer greater than or equal to one, of the wavelengths .lambda..sub.1 through .lambda..sub.n are coupled to k T switches 2' via an nxk S-switch 4. Outputs of the T switches 2' are coupled to n time division highways of the wavelengths .lambda..sub.1 through .lambda..sub.n via a kxn S-switch 5.
FIG. 3 shows a more detailed block diagram of the conventional optical switching system. A variable branching filter 1" branches a reference light in which a light having an accurate wavelength is wavelength-division multiplexed. The wavelength conversion is made by modulating outputs of the variable branching filter 1" in optically controlled optical modulators 3' depending on outputs of optical T-switches 2". By switching the selecting wavelengths of the variable branching filter 1" for every time slot, it is possible to also vary the wavelength for every time slot after the wavelength conversion.
The input optical signal which is a wavelength-division multiplexed and time-division multiplexed signal is conventionally subjected to the arbitrary interchange of the wavelength and the time slot in the above described manner. However, the conventional optical switching system is not practical in that it lacks flexibility in the interchange of information contents, collection of required information and the like, and a highly satisfactory service cannot be expected when applied to the information interchange services.